The invention relates to a method of imaging a specimen using ptychography, whereby a charged-particle beam is directed from a source through an illuminator so as to traverse the specimen and land upon a detector, an output of the detector being used in combination with a mathematical reconstruction technique so as to calculate at least one property of a charged-particle wavefront exiting the specimen.
The invention additionally relates to an apparatus for performing such a method.
The invention further relates to a charged-particle microscope in which such a method can be enacted and/or in which such an apparatus can be comprised.
Charged-particle microscopy is a well-known and increasingly important technique for imaging microscopic objects, particularly in the form of electron microscopy. Historically, the basic genus of electron microscope has undergone evolution into a number of well-known apparatus species, such as the Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), and Scanning Transmission Electron Microscope (STEM), and also into various sub-species, such as so-called “dual-beam” tools (eg. a FIB-SEM), which additionally employ a “machining” Focused Ion Beam (FIB), allowing supportive activities such as ion-beam milling or Ion-Beam-Induced Deposition (IBID), for example. More specifically:                In a SEM, irradiation of a specimen by a scanning electron beam precipitates emanation of “auxiliary” radiation from the specimen, in the form of secondary electrons, backscattered electrons, X-rays and cathodoluminescence (infrared, visible and/or ultraviolet photons), for example; one or more components of this emanating radiation is/are then detected and used for image accumulation purposes.        In a TEM, the electron beam used to irradiate the specimen is chosen to be of a high-enough energy to penetrate the specimen (which, to this end, will generally be thinner than in the case of a SEM specimen); the transmitted electrons emanating from the specimen can then be used to create an image. When such a TEM is operated in scanning mode (thus becoming a STEM), the image in question will be accumulated during a scanning motion of the irradiating electron beam.More information on some of the topics elucidated here can, for example, be gleaned from the following Wikipedia links:            http://en.wikipedia.org/wiki/Electron_microscope    http://en.wikipedia.orgiwiki/Scanning_electron_microscope    http://en.wikipedia.org/wiki/Transmission_electron_microscopy    http://en.wikipedia.org/wiki/Scanning_transmission_electron_microscopyAs an alternative to the use of electrons as irradiating beam, charged particle microscopy can also be performed using other species of charged particle. In this respect, the phrase “charged particle” should be broadly interpreted as encompassing electrons, positive ions (e.g. Ga or He ions), negative ions, protons and positrons, for instance. As regards non-electron-based charged particle microscopy, some further information can, for example, be gleaned from references such as the following:    https://en.wikipedia.org/wiki/Focused_ion_beam    http://en.wikipedia.org/wiki/Scanning_Helium_Ion_Microscope    W. H. Escovitz, T. R. Fox and R. Levi-Setti, Scanning Transmission Ion Microscope with a Field Ion Source, Proc. Nat. Acad. Sci. USA 72(5), pp 1826-1828 (1975).    http://www.ncbi.nlm.nih.gov/pubmed/22472444It should be noted that, in addition to imaging and performing (localized) surface modification (e.g. milling, etching, deposition, etc.), a charged particle microscope may also have other functionalities, such as performing spectroscopy, examining diffractograms, etc.
In all cases, a Charged-Particle Microscope (CPM) will comprise at least the following components:                A radiation source, such as a Schottky electron source or ion gun.        An illuminator, which serves to manipulate a “raw” radiation beam from the source and perform upon it certain operations such as focusing, aberration mitigation, cropping (with an aperture), filtering, etc. It will generally comprise one or more (charged-particle) lenses, and may comprise other types of (particle-) optical component also. If desired, the illuminator can be provided with a deflector system that can be invoked to cause its exit beam to perform a scanning motion across the specimen being investigated.        A specimen holder, on which a specimen under investigation can be held and positioned (e.g. tilted, rotated). If desired, this holder can be moved so as to effect scanning motion of the beam w.r.t. the specimen. In general, such a specimen holder will be connected to a positioning system.        A detector (for detecting radiation emanating from an irradiated specimen), which may be unitary or compound/distributed in nature, and which can take many different forms, depending on the radiation being detected. Examples include photodiodes, CMOS detectors, CCD detectors, photovoltaic cells, X-ray detectors (such as Silicon Drift Detectors and Si(Li) detectors), etc. In general, a CPM may comprise several different types of detector, selections of which can be invoked in different situations.In the case of a transmission-type microscope (such as a (S)TEM, for example), a CPM will specifically comprise:        An imaging system, which essentially takes charged particles that are transmitted through a specimen (plane) and directs (focuses) them onto analysis apparatus, such as a detection/imaging device, spectroscopic apparatus (such as an EELS device), etc. As with the illuminator referred to above, the imaging system may also perform other functions, such as aberration mitigation, cropping, filtering, etc., and it will generally comprise one or more charged-particle lenses and/or other types of particle-optical components.In what follows, the invention may—by way of example—sometimes be set forth in the specific context of electron microscopy; however, such simplification is intended solely for clarity/illustrative purposes, and should not be interpreted as limiting.        
A method as set forth in the opening paragraph above is, for example, elucidated in an article by M. J. Humphry et al. in Nature Communications, 3:730, DOI 10:1038/ncomms1733, Macmillan Publishers Limited (2011), see:    http://www.nature.com/ncomms/journal/v3/n3/full/ncomms1733.htmlThis article discusses electron beam ptychography, and its application to conduct a form of “lens-less microscopy” in a SEM. The approach disclosed in the article can be regarded as a modification of related techniques from the field of X-ray imaging, where lens-less techniques are attractive because of the difficulty in producing satisfactory X-ray optical systems. In the article, an illuminator produces a convergent electron beam that is focused onto and through a specimen (in a specimen plane) so as to impinge upon a CCD detector. The convergent electron beam is referred to as a “probe”, and this probe is scanned across the specimen in a series of measurement sessions. In each such session, an associated diffraction pattern is recorded by the CCD, and these various patterns are used as input to an iterative mathematical inverse problem solving technique in an attempt to reconstruct the amplitude/phase of the electron-beam wavefront exiting the specimen (somewhat analogous to a deconvolution technique). This, in turn, provides information on the structure of the employed specimen. For more information on this approach, see, for example, the article by J. M. Rodenburg and H. M. L. Faulkner in Appl. Phys. Lett. 85, pp. 4795-4798 (2004) [see the following link]:    http://scitation.aip.org/content/aip/journal/apl/85/20/10.1063/1.1823034Some general information on ptychography can be gleaned from the following Wikipedia link:    https://en.wikipedia.org/wiki/PtychographyAnd an example of a specific ptychographic method/apparatus is set forth in U.S. Pat. No. 9,202,670 (with the same assignee as the present patent application), which is incorporated herein by reference.
For good order, it is pointed out that what is effectively being reconstructed in such ptychography techniques is a change in a (planar) wavefront as it traverses the specimen. Although ptychography techniques use a relative narrow beam that only illuminates a localized area of the specimen (and that is moved to a series of different locations on the specimen in a corresponding series of measurement sessions), the reconstruction effectively calculates changes to a (virtual) broad wavefront that traverses the full area of the specimen in one go. This point will be fully grasped by the skilled artisan.
A problem with current ptychographic techniques is that they are subject to so-called “phase wrapping” (also called “phase vortexing”)—which can be particularly problematic for specimens that are relatively thick and/or comprise material with a relatively high atomic number. This phenomenon has to do with the fact that conventional wavefront reconstruction techniques intrinsically limit the reconstructed wavefront phase to a truncated range [0, 2π] (or, equivalently, [−π, +π]); in reality, however, the actual phase can have a value outside this range, in which case the phase will have to be “wrapped up” in order to fit within the truncated range. When the reconstructed phase is used to produce an image of the specimen, such “phase wrapping” can cause distortion of the image (see, for example, FIG. 2A); in order to (attempt to) restore such a distorted image, a “phase unwrapping” algorithm must be applied.